The production and processing of plastics is characterized by constantly increasing demands on the quality of individual products. To meet these quality requirements, it is necessary to keep precisely to predetermined specifications particularly in regard to the concentration of individual constituents in plastic mixtures. In the context of the invention, plastic mixtures may be inter alia copolymers, polymer blends or filler- or fiber-reinforced plastics. Another field of application of the invention is the determination of additive concentrations. The additives in question are, for example, mold release agents or plasticizers. The concentrations of mold release agents are typically less than 1%. Nevertheless, their exact percentage content by weight in the plastic mixture often has to be determined.
There are already various known spectroscopic processes for determining the composition of plastic mixtures. In one frequently used process, a solution of the plastic is initially prepared and a transmission spectrum of the solution is then run. On the basis of absorption bands which are characteristic of the components to be determined, the concentrations of those components can be determined after calibration using the Lambert-Beer law or more recent evaluation methods. However, this process is attended by the disadvantage that there are no known solvents for certain modern high-performance plastics, for example polyphenylene sulfide, at temperatures below 150.degree. C.
In addition, U.S. Pat. No. 4,717,827 describes a process in which the transmission measurement of the plastic to be analyzed is carried out on a molten sample. However, this process is suitable above all for the on-line production control of a product, but less suitable for the measurement of a number of different samples in a short time because cleaning and refilling of the measuring cell are relatively complicated. The most advantageous method of analyzing a number of different samples is to determine the transmission spectra of pressed films of the plastic mixtures to be analyzed. Pressed films are obtained in known manner by the compression-molding of a plastic mixture at a compression mold temperature above the melting temperature of the mixture to be analyzed. Their thickness is typically between 1 and 500 m. In addition, test specimens produced by injection molding with typical wall thicknesses of 0.1 to 100 mm may also be used for spectroscopic measurements. Since both pressed films or injection moldings and also the analysis beam of commercially available spectrometers often show an anisotropies or inhomogeneities, the spectra of these plastic films are subject to considerable variations which have a correspondingly adverse effect on the accuracy of the concentration measurement of individual components in the plastic mixture.
The problem addressed by the present invention was to provide a process for the analysis of plastic mixtures with increased accuracy by transmission spectroscopy using solid samples.